4-pin vs 3-pin for Fan on Mobo

[wpcoe]

My Gigabyte P35 mobo has 4 fan headers: two 4-pin (marked CPU_FAN and SYS_FAN2) and two 3-pin (marked SYS_FAN1 and PWR_FAN).

Of course, I had my HSF fan plugged in to the 4-pin CPU_FAN, and the rest of my three 3-pin fans randomly to the other headers.

Then, I bought a CoolerMaster Hyper 212+ and when I plugged it into the CPU_FAN header, it didn't work. The shop where I bought it showed me that if I turned off fan speed control in BIOS, the CM H212+ fan would speed along at 2000rpm. However, I wanted speed control, and the tech there said "not possible," and offered a refund. As a compromise, he showed me if I plugged it into the other 4-pin SYS_FAN2 header the fan would spin slower, but at home tests showed in only operated on that header in a narrow 500-600rpm range even at high load, running Intel BurnTest.

So, then the experimentation began and I realized that my other 3-pin fans would be speed-controlled on the 4-pin headers. WTF? I thought the purpose of the 4th pin was fan speed control.

How does a 3-pin fan get speed control from a 4-pin header?

I put a second (3-pin, 1500rpm) fan on the CM H212+ for a push-pull configuration. Both the push (the CM fan that came with the HS) and the pull are quiet enough at full speed (I can't even hear them) that I switched them both to 3-pin mobo headers and let them blaze away at full speed.

Is there a downside to having those two push/pull HS fans running at full speed continuously (push=2000rpm and pull=1500rpm)?

FYI, I put the 8cm mobo rear exhaust fan and the 12cm side exhaust fan on the 4-pin headers and they -- mysteriously to me -- operate slowly at idle/light-load and spring to life when needed.

 

[Tofurkeymeister]

There are two ways the motherboard can control fan speed AFAIK: Voltage, and Pulse Width Modulation (PWM).

Voltage simply varies the voltage applied to the fan. Lower voltage = slower fan speed. As it is simply augmenting the main power source, voltage control works with both three and four pins sockets.

PWM utilized the fourth pin to send a signal about what the fan speed should be. The actual power source comes from the motherboard untouched.

Hence, if you plug ANY fan into a ANY socket supporting voltage control, you can control its speed, as only three pins are used.

If your motherboard does not support this kind of control, you are only able to control fan speed by plugging a 4-pin fan into a 4-pin socket.

P.S. You might be able to switch between Voltage and PWM in the BIOS.

 

[wpcoe]

Thanks!

I just swapped mobo's (and CPU and HDD and video cards) so I'm a bit overwhelmed right now. The BIOS screens for the new mobo are LOADED with options, so I'll check about the PWM vs Voltage setting. I do know that my recently deceased mobo did have those settings under "smart fan control," but I had no idea what they meant.

 

[Tofurkeymeister]

If you have a Gigabyte P35 mobo, Its under PC Health status. (Mess with smart fan control)

 

[wpcoe]

Thanks for the BIOS location tip. I'm away from home for another week or so, but when I get home I will experiment with that (as well as lots of the other settings in BIOS!).

I'll take this opportunity to repeat a question I had above: Is there a downside to having those two push/pull HS fans running at full speed continuously (push=2000rpm and pull=1500rpm)? i.e. Is it okay to have a slower speed fan at the rear pulling the air (or should I reverse them to have the push fan the slower one?), and is there a problem just letting those two fans spin at top speed, since noise is not an issue?

[edited to add:] Just found this comment in another thread (http://forums.anandtech.com/showthread.php?p=30219529):

"I understand that for a CPU fan faster is not better that too much flow you dont "absorb" the heat from the cooler."

Is that correct? Am I losing cooling capacity by letting the HS fans run at at max speed?